1. Field of the Invention
The present invention relates to a lightweight magnesia clinker and a process for the preparation of the same. More particularly, the present invention relates to a lightweight magnesia clinker which is effectively used as a material for coating a surface of a lining brick employable for a ladle for receiving molten steel or a tundish and as a material of a basic heat-insulating brick in the field of steelmaking industry, and a process for the preparation of the same.
2. Description of Prior Art
In the steelmaking art, a basic fire-resistant material has been heretofore employed as a lining material of a converter or a ladle, and recently the basic fire-resistant material has been also used widely as a coating material of a tundish in a recently employed continuous casting process.
The coating material of a tundish is required to have various properties such as resistance to smelting loss by slag and resistance to permeability of molten steel as well as heat resistance and heat insulation. Further, since a coating material is generally used in a large amount, economically advantageous is a coating material having a larger volume per unit weight, namely, a coating material having a relatively small bulk specific gravity. The conventional coating material is generally made of a hard magnesia clinker. The hard magnesia clinker is prepared through firing at a high temperature and is in the fired state, so that such magnesia clinker has a small apparent porosity and a relatively high bulk specific gravity such as a bulk specific gravity of 3.0 to 3.3. Accordingly, when the hard magnesia clinker is used for preparing a fire-resistant material such as the above-mentioned heat-insulating material and coating material, a variety of measures are practically adopted in the preparation of the fire-resistant material. However, known measures adopted in the process using the hard magnesia clinker hardly give satisfactory fire-resistant materials. Employment of a lightweight magnesia clinker is expected to make it possible to readily provide a lightweight fire-resistant material showing high heat-insulation.
There are known various processes for preparing a magnesia clinker having a high apparent porosity. For example, Japanese Patent Provisional Publication No. 59(1984)-190256 discloses a process for the preparation of a porous magnesia clinker which comprises the steps of mixing a magnesium component and an oil coke, granulating the obtained mixture into desired particles, then calcining the particles to burn out the oil coke so as to form pores within the calcined particles, and further firing the particles at a high temperature.
The porous magnesia clinker obtained in the above-described process is characterized in that the pore size is made larger in order to provide a higher apparent porosity to the resulting magnesia clinker. Accordingly, the magnesia clinker prepared by the process still has drawbacks, although it it improved only in the heat insulation and the bulk specific gravity. For example, when the magnesia clinker is used as a principal ingredient of the above-mentioned coating material, slag or molten steel easily permeates the clinker because of the presence of the large sized pores, whereby the clinker is broken or the molten steel is stained. Moreover, as the pore size is made larger, the resulting magnesia clinker decreases in the mechanical strength.
For those reasons, it is desired to prepare a lightweight magnesia clinker having not only a higher apparent porosity but also smaller sized pores.